With the digitization of information, it is possible to obtain, in the digital form, information of various types such as video, audio and data. For the purpose of copyright protection, however, a corresponding key is needed when people are trying to attain such information from certain media (for example, Internet, or physical storage media such as optical or magnetic disks). Such a process is known in the art as “Digital Right Management” (DRM). In DRM, said information is called digital content or digital asset, and each of said digital content is protected by a content key CK.
A hierarchical content key management system is proposed in Chinese Application No. 03123222.1, filed on Apr. 22, 2003, the entirety of which is incorporated herein for reference. In this hierarchical content key management system, it is assumed that each content should have a unique position in the key tree, and this unique position corresponds to an authorization key AK. In other words, each content should have a content name (or content path) that can uniquely identify the position of this content in the hierarchical key tree. Said system functions well in cases where one content corresponds to one position (i.e., where one content has only one authorization key AK).
When the system is put into practical use, however, it is realized that one content is often to be placed at a plurality of positions. For example, the film Star War II might be in the path “Star War Series.backslash. Star War II”, or it might be in the path “Movies.backslash.US.backslash.2002.backslash.Star War II”. In this case, for the afore-mentioned hierarchical content key management system, on one hand, the position determines the authorization key, then one content will have two different keys, on the other hand, it is desired that possession of either key will suffice for decryption of the content. Obviously, these two aspects are contradictory.
One simple way to solve this problem of multi-path (or multi-name, multi-position) content is to make multiple different encrypted copies of the content, with each copy having an authorization key respectively. This, however, will consume massive amount of storage resources, and brings great difficulty for content management.